The present invention relates to a clutch for a power transmission system for a motor vehicle, and more particularly to a clutch in the form of a fluid operated multiple-disk friction clutch. This type of clutch is employed, for example in a transfer device for a four-wheel drive vehicle, for controlling the torque capacity in accordance with driving conditions of the vehicle.
As shown in FIG. 4, a conventional fluid operated multiple-disk friction clutch 100 comprises a drive drum 101 having a hub 104' secured to a drive shaft 104, a driven drum 113 secured to a gear 114. An oil chamber 103 is formed between drum 101 and a piston 102. When oil is supplied to the chamber 103 through passages 105 and 105' formed in the shaft 104 and hub 104', the piston 102 is urged by the oil pressure to push disks 107 secured to the piston to disks 108 secured to the hub 113 against the elastic force of a spring 106. Thus, the disks 107 and 108 are engaged to transmit the torque of the shaft 104 to the gear 114. When the oil is drained from the chamber 103, the piston 102 is returned to the position of FIG. 4 by the spring 106, so that the disks 107 and 108 are disengaged.
In high speed operation of the clutch, the centrifugal force in the oil in the oil chamber 103 is exerted on the piston 102. In order to avoid the influence of the force, a drain passage 109 having a check ball 110 is formed in the piston 102. When the rotational speed exceeds a predetermined speed, the check ball 110 moves away from a port 111 of the drain passage 109 by the centrifugal force as shown in FIG. 5, so that the oil is drained from the port 111 for cancelling the centrifugal oil pressure.
If the conventional clutch is used for a transfer clutch of an automatic transmission, the transfer clutch is provided for controlling the torque capacity by continuously varying the actuating pressure. In such a system, it is difficult to control continuously from a low speed to a high speed, because the oil is drained in a high speed range. In addition, when the speed increases more than the predetermined speed, there is no oil in a space between the drain port 111 and the chamber 103. Accordingly, timing of the clutch engagement at a low speed is different with that of a high speed operation. Further, it is possible to produce a negative pressure in the space.
Japanese patent application laid-open No. 55-57728 discloses a clutch in which another oil chamber is provided on opposite side of a piston corresponding to an actuating oil chamber for cancelling the centrifugal oil pressure. However, in such a structure, a necessary hydraulic control structure besides the oil chamber must be provided, which renders the clutch complicated in construction.